Peroxide-crosslinkable compositions and processes for their manufacture

ABSTRACT

A peroxide-crosslinkable composition comprising:
         (A) A peroxide-crosslinkable polymer, e.g., a polyethylene;   (B) A nitrogenous base, e.g., a low molecular weight, or low melting, or liquid nitrogenous base such as triallyl cyanurate (TAC); and   (C) One or more antioxidants (AO), e.g., distearylthiodipropionate (DSTDP).       

     The composition is useful in the manufacture of insulation sheaths for high and extra high voltage wire and cable.

FIELD OF THE INVENTION

The invention relates to peroxide-crosslinkable compositions. In oneaspect, the invention relates to processes of preparingperoxide-crosslinkable compositions while in another aspect, theinvention relates to insulation made from the compositions that isuseful in the manufacture of high voltage (HV) or extra high voltage(EHV) power cable.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,656,986 teaches various polyethylene,peroxide-crosslinkable compositions useful in the manufacture of powercable insulation. Some of these compositions have achieved commercialsuccess in the medium voltage power cable market, and an interest existsin extending these commercially successful compositions into the highand extra high voltage power cable markets.

The manufacture of power cable insulation is a multistep process thatcan be separated into two broad parts, i.e., first making a compositionfrom which the cable insulation is made, and second, extruding thecomposition over single or stranded conductor as an insulation.

In one embodiment of the first part of the process, i.e., the part inwhich the composition is made, a base polymer, e.g., polyethylene, ismixed with one or more additives and then formed into pellets which aresoaked with peroxide and subsequently stored and/or shipped to afabricator who performs the second part of the process, i.e., convertingthe pellets to a wire or cable coating. To avoid acid catalyzeddecomposition of peroxide during storage and shipping, U.S. Pat. No.6,656,986 teaches inclusion of oligomeric and/or high molecular weighthindered amine stabilizers (HAS).

In the making of the pellets, care is taken not to introduce or createimpurities that can adversely affect the utility of the composition onceformed into a wire or cable sheath. However, some impurities areinevitably introduced into the composition either as, for example,contaminants associated with feed materials to the process, or are madeduring the process as, for example, gels that result from degradation ofthe base polymer. Efforts are made, of course, to minimize and removethese impurities before the composition is extruded as a power cablesheath. Some of the impurities are in the form of fine, e.g., less than100 microns (μm), particulates and are susceptible to removal from thecomposition by filtering. In those embodiments in which the compositionis compounded within an extruder, a fine-mesh screen is typicallylocated at or near the die head of the extruder such that the meltwithin the extruder must pass through the screen before it leaves theextruder. As the filter becomes plugged with particulates, pressurebuilds within the extruder and the operational efficiency of theextruder drops until the filter is cleansed or replaced. In thoseembodiments in which an oligomeric or high molecular weight base, e.g.,a oligomeric or high molecular weight HAS, is present in the compositionprior to melt filtration, it tends to contribute to the plugging of thescreen and diminishing the operational efficiency of the extruder andoverall run efficiency of the process.

Insulation for use in medium voltage power cable applications cantypically tolerate more impurities than those for use in high or extrahigh voltage power cable applications. As such, the screen used tofilter the composition before extrusion into pellets can be more coarse,i.e., have a larger openings, than that used for filtering compositionsfor use in high or extra high voltage power cable applications. As aconsequence and all else being equal, the finer (smaller) the screenmesh through which a melt must pass, the more particulate it will trap,the faster it will plug, and the shorter the time interval will bebetween filter cleaning and/or replacement. This, in turn, affects theoperational efficiency of the compounding process.

Of particular interest to the extension of compositions currentlydesigned for use in medium voltage power cable applications to high andextra high voltage power cable applications is the reduction and/orelimination of particulate contaminants and gels during the compoundingof the base polymer with additives and/or fillers and to the extent thatsuch gels are made, their removal by filtering before the composition isfabricated into pellets. Further to this interest is maintaining therelative stability of the pellet against loss of crosslinking efficiencyduring shipping and/or storage, and the minimizing of water generationduring cure.

SUMMARY OF THE INVENTION

In one embodiment the invention is an additive pre-blend compositioncomprising or consisting essentially of 0.6 to 66 weight percenttriallyl cyanurate (TAC) and 34 to 99.4 wt % of an antioxidant (AO). Inone embodiment the invention is an additive pre-blend compositioncomprising or consisting essentially of TAC and AO at a TAC:AO weightratio of 1:100 to 3:2.

In one embodiment the invention is a peroxide-crosslinkable compositioncomprising:

-   -   (A) A peroxide-crosslinkable polymer;    -   (B) A peroxide;    -   (C) Triallyl cyanurate (TAC); and    -   (D) One or more antioxidants (AO),        the TAC and AO present at a TAC:AO weight ratio of 1:100 to 3:2        wherein TAC is present at less than (<) 0.01% in the final        formulation. In one embodiment the composition is in the form of        a pellet.

In one embodiment the invention is a process for making aperoxide-crosslinkable pellet, the process comprising the steps of:

-   -   (1) Forming a homogeneous melt of a:        -   (A) Peroxide-crosslinkable polymer; and        -   (B) Pre-blend of triallyl cyanurate (TAC) and AO, the            pre-blend comprising or consisting essentially of 0.6 to 66            wt % TAC and 34 to 99.4 wt % of AO;    -   (2) Passing the homogeneous melt of (1) through a filter with a        mesh size of less than 100 μm; and    -   (3) Forming pellets from the filtered homogeneous melt of (2).        In a further step, the pellets are impregnated with a peroxide,        typically by spraying or otherwise applying a liquid peroxide to        the pellets and allowing the pellets to absorb the peroxide. In        one embodiment the peroxide is mixed with an organic nitrogenous        base, e.g., TAC, before the peroxide is applied to the pellets.

In one embodiment the invention is a peroxide-crosslinkable compositioncomprising:

-   -   (A) A peroxide-crosslinkable polymer;    -   (B) A low molecular weight, or low melting, or liquid        nitrogenous base at 0.0005% to 0.01%;    -   (C) One or more antioxidants (AO); and    -   (D) Optionally, a peroxide.        In one embodiment the base comprises TAC. In one embodiment the        base comprises        N,N′-1,6-hexanediylbis(N-(2,2,6,6-tetramethyl-4-piperidinyl)-formamide        (UVINUL™ 4050 from BASF) and/or TINUVIN™ 765 (mixture of        bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate and methyl        1,2,2,6,6-pentamethyl-4-piperidyl sebacate). In one embodiment        the antioxidant contains sulfur, e.g., distearylthiodipropionate        (DSTDP).

In one embodiment the invention is a process for making aperoxide-crosslinkable pellet, the process comprising the steps of:

-   -   (1) Forming a homogeneous melt of:        -   (A) A peroxide-crosslinkable polymer;        -   (B) A low molecular weight, or liquid, or low melting            nitrogenous base at 0.0005% to 0.01%, and        -   (C) Optionally, an antioxidant (AO);    -   (2) Passing the homogeneous melt of (1) through a filter with a        mesh size of less than 100 μm; and    -   (3) Forming pellets from the filtered homogeneous melt of (2).        In a further step, the pellets are impregnated with a peroxide,        typically by spraying or otherwise applying a liquid peroxide to        the pellets and allowing the pellets to absorb the peroxide. In        one embodiment the peroxide is mixed with an organic nitrogenous        base, e.g., TAC, before the peroxide is applied to the pellets.

In one embodiment the invention is a process for making aperoxide-crosslinkable pellet, the process comprising the steps of:

-   -   (1) Forming a melt of a peroxide-crosslinkable polymer with AO;    -   (2) Passing the melt of (1) through a filter with a mesh size of        less than 100 μm;    -   (3) Forming pellets from the filtered homogeneous melt of (2);        and    -   (4) Impregnating the pellets with a low molecular weight, or        liquid nitrogenous base.        In one embodiment the pellets are also impregnated with a        peroxide.

In one embodiment the invention is a wire or cable comprising a sheathmade from the pellets of any of the embodiments described above. In oneembodiment the wire or cable is a high voltage or extra-high voltagewire or cable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart reporting the thermograms for the Inventive Example 1TAC, AO mixture, and pre-blend of TAC and the AO mixture at a 1:1 weightratio.

FIG. 2 is a chart reporting the thermograms for the Inventive Example 1TAC, AO mixture, and pre-blend of TAC and the AO mixture at a 2:98weight ratio.

FIG. 3 is a chart reporting the thermograms for the Comparative ExampleTAC, tetradecane, and pre-blend of TAC and tetradecane mixture at a 1:1weight ratio.

FIG. 4 is graph reporting a comparison of peroxide stability in LDPEbetween a sample compounded with TINUVIN™ 622 and impregnated (soaked)with TINUVIN™ 622.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Definitions

For purposes of U.S. patent practice, all patents, patent applicationsand other cited documents within this application are incorporated intheir entirety herein by reference to the extent that they are not inconflict with the disclosure of this application.

The numerical ranges in this disclosure are approximate, and thus mayinclude values outside of the range unless otherwise indicated.Numerical ranges include all values from and including the lower and theupper values, in increments of one unit, provided that there is aseparation of at least two units between any lower value and any highervalue. As an example, if a compositional, physical or other property,such as, for example, molecular weight, viscosity, melt index, etc., isfrom 100 to 1,000, it is intended that all individual values, such as100, 101, 102, etc., and sub ranges, such as 100 to 144, 155 to 170, 197to 200, etc., are expressly enumerated. For ranges containing valueswhich are less than one or containing fractional numbers greater thanone (e.g., 1.1, 1.5, etc.), one unit is considered to be 0.0001, 0.001,0.01 or 0.1, as appropriate. For ranges containing single digit numbersless than ten (e.g., 1 to 5), one unit is typically considered to be0.1. These are only examples of what is specifically intended, and allpossible combinations of numerical values between the lowest value andthe highest value enumerated, are to be considered to be expresslystated in this disclosure. Numerical ranges are provided within thisdisclosure for, among other things, the amount of a particular componentin a composition.

“Comprising”, “including”, “having” and like terms mean that thecomposition, process, etc. is not limited to the components, steps, etc.disclosed, but rather can include other, undisclosed components, steps,etc. In contrast, the term “consisting essentially of” excludes from thescope of any composition, process, etc. any other component, step etc.excepting those that are not essential to the performance, operabilityor the like of the composition, process, etc. The term “consisting of”excludes from a composition, process, etc., any component, step, etc.not specifically disclosed. The term “or”, unless stated otherwise,refers to the disclosed members individually as well as in anycombination.

“Wire” and like terms mean a single strand of conductive metal, e.g.,copper or aluminum, or a single strand of optical fiber.

“Cable” and like terms mean at least one wire or optical fiber within asheath, e.g., an insulation covering or a protective outer jacket.Typically, a cable is two or more wires or optical fibers boundtogether, typically in a common insulation covering and/or protectivejacket. The individual wires or fibers inside the sheath may be bare,covered or insulated. Combination cables may contain both electricalwires and optical fibers. The cable, etc. can be designed for low,medium, high and extra high voltage applications. Low voltage cables aredesigned to carry less than 3 kilovolts (kV) of electricity, mediumvoltage cables 3 to 69 kV, high voltage cables 70 to 220 kV, and extrahigh voltage cables excess of 220 kV. Typical cable designs areillustrated in U.S. Pat. Nos. 5,246,783, 6,496,629 and 6,714,707.

“Conductor”, “electrical conductor” and like terms mean an object whichpermits the flow of electrical charges in one or more directions. Forexample, a wire is an electrical conductor that can carry electricityalong its length. Wire conductors typically comprise copper or aluminum.

“Shipping and storage conditions” and like terms mean the temperature,pressure and humidity at which the compositions of this invention,typically in the form of pellets, are shipped from manufacturer toend-user and under which the compositions are held prior to and/or aftershipping. Temperatures range from subfreezing (in cold climates) toabove 40° C. in an un-air conditioned warehouse in a hot climate.Humidity can range from 0 to 100 percent, and pressure is typicallyatmospheric.

“Melt” and like terms mean a solid composition in a molten state. A meltmay or may not comprise gels and/or solid particulates.

“Gel” and like terms mean a crosslinked polymer, typically in acolloidal state. Gels can vary in size, molecular weight, structure andcomposition.

“Melt filtration” and like terms mean passing a melt through a filter orscreen to remove one or more gels and/or solid particulates from themelt.

“Peroxide-crosslinkable polymer” and like terms mean a polymer, e.g., apolyolefin such as polyethylene, that can be crosslinked undercrosslinking conditions, e.g., at a temperature of 160° C. to 250° C.,through a free radical mechanism that is initiated by a peroxide, e.g.,dicumyl peroxide.

Peroxide-Crosslinkable Polymer

Although any polymer that can be crosslinked by a peroxide-initiatedreaction can be used as the peroxide-crosslinkable polymer in thepractice of this invention, typically and preferably the polymer is apolyolefin, and more typically and preferably a polyethylene. As thatterm is used here, polyethylene is a homopolymer of ethylene or acopolymer of ethylene and a minor proportion of one or morealpha-olefins having 3 to 12 carbon atoms, and preferably 4 to 8 carbonatoms, and, optionally, a diene, or a mixture or blend of suchhomopolymers and copolymers. The mixture can be a mechanical blend or anin situ blend. Examples of the alpha-olefins are propylene, 1-butene,1-hexene, 4-methyl-1-pentene, and 1-octene. The polyethylene can also bea copolymer of ethylene and an unsaturated ester such as a vinyl ester,e.g., vinyl acetate or an acrylic or methacrylic acid ester.

The polyethylene can be homogeneous or heterogeneous. The homogeneouspolyethylenes usually have a polydispersity (Mw/Mn) in the range ofabout 1.5 to about 3.5 and an essentially uniform comonomerdistribution, and are characterized by single and relatively lowdifferential scanning calorimetry (DSC) melting points. Theheterogeneous polyethylenes, on the other hand, have a polydispersity(Mw/Mn) greater than 3.5 and do not have a uniform comonomerdistribution. Mw is defined as weight average molecular weight, and Mnis defined as number average molecular weight. The polyethylenes canhave a density in the range of 0.860 to 0.950 gram per cubic centimeter(g/cc), and preferably have a density in the range of 0.870 to about0.930 g/cc. They also can have a melt index in the range of about 0.1 toabout 50 grams per 10 minutes.

The polyethylenes can be produced by low or high pressure processes.They can be produced in the gas phase, or in the liquid phase insolutions or slurries by conventional techniques. Low pressure processesare typically run at pressures below 1000 psi whereas high pressureprocesses are typically run at pressures above 15,000 psi.

Typical catalyst systems, which can be used to prepare thesepolyethylenes, are magnesium/titanium based catalyst systems, which canbe exemplified by the catalyst system described in U.S. Pat. No.4,302,565 (heterogeneous polyethylenes); vanadium based catalyst systemssuch as those described in U.S. Pat. No. 4,508,842 (heterogeneouspolyethylenes) and U.S. Pat. Nos. 5,332,793; 5,342,907; and 5,410,003(homogeneous polyethylenes); a chromium based catalyst system such asthat described in U.S. Pat. No. 4,101,445; a metallocene catalyst systemsuch as those described in U.S. Pat. Nos. 4,937,299, 5,272,236,5,278,272, and 5,317,036 (homogeneous polyethylenes); or othertransition metal catalyst systems. Many of these catalyst systems areoften referred to as Ziegler-Natta catalyst systems or Phillips catalystsystems. Catalyst systems, which use chromium or molybdenum oxides onsilica-alumina supports, can be included here. Typical processes forpreparing the polyethylenes are also described in the aforementionedpatents. Typical in situ polyethylene blends and processes and catalystsystems for providing same are described in U.S. Pat. Nos. 5,371,145 and5,405,901. The various polyethylenes can include low densityhomopolymers of ethylene made by high pressure processes (HP-LDPE),linear low density polyethylenes (LLDPE), very low density polyethylenes(VLDPE), medium density polyethylenes (MDPE), high density polyethylene(HDPE) having a density greater than 0.940 g/cc and metallocenecopolymers with densities less than 0.900 g/cc. The latter fivepolyethylenes are generally made by low pressure processes. Aconventional high pressure process is described in Introduction toPolymer Chemistry, Stille, Wiley and Sons, New York, 1962, pages 149 to151. The high pressure processes are typically free radical initiatedpolymerizations conducted in a tubular reactor or a stirred autoclave.In the stirred autoclave, the pressure is in the range of about 10,000to 30,000 psi and the temperature is in the range of about 175 to about250° C., and in the tubular reactor, the pressure is in the range ofabout 25,000 to about 45,000 psi and the temperature is in the range ofabout 200 to about 350° C. Blends of high pressure polyethylene andmetallocene resins are particularly suited for use in the application,the former component for its excellent processability and the latter forits flexibility.

Copolymers comprised of ethylene and unsaturated esters are well known,and can be prepared by the conventional high pressure techniquesdescribed above. The unsaturated esters can be alkyl acrylates, alkylmethacrylates, and vinyl carboxylates. The alkyl group can have 1 to 8carbon atoms and preferably has 1 to 4 carbon atoms. The carboxylategroup can have 2 to 8 carbon atoms and preferably has 2 to 5 carbonatoms. The portion of the copolymer attributed to the ester comonomercan be in the range of about 5 to about 50 percent by weight based onthe weight of the copolymer, and is preferably in the range of about 15to about 40 percent by weight. Examples of the acrylates andmethacrylates are ethyl acrylate, methyl acrylate, methyl methacrylate,t-butyl acrylate, n-butyl acrylate, n-butyl methacrylate, and2-ethylhexyl acrylate. Examples of the vinyl carboxylates are vinylacetate, vinyl propionate, and vinyl butanoate. The melt index of theethylene/unsaturated ester copolymers can be in the range of about 0.5to about 50 grams per 10 minutes, and is preferably in the range ofabout 2 to about 25 grams per 10 minutes. One process for thepreparation of a copolymer of ethylene and an unsaturated ester isdescribed in U.S. Pat. No. 3,334,081.

The VLDPE can be a copolymer of ethylene and one or more alpha-olefinshaving 3 to 12 carbon atoms and preferably 3 to 8 carbon atoms. Thedensity of the VLDPE can be in the range of 0.870 to 0.915 g/cc. It canbe produced, for example, in the presence of (i) a catalyst containingchromium and titanium, (ii) a catalyst containing magnesium, titanium, ahalogen, and an electron donor; or (iii) a catalyst containing vanadium,an electron donor, an alkyl aluminum halide modifier, and a halocarbonpromoter. Catalysts and processes for making the VLDPE are described,respectively, in U.S. Pat. Nos. 4,101,445; 4,302,565; and 4,508,842. Themelt index of the VLDPE can be in the range of about 0.1 to about 20grams per 10 minutes (g/10 min) and is preferably in the range of about0.3 to about 5 g/10 min. The portion of the VLDPE attributed to thecomonomer(s), other than ethylene, can be in the range of about 1 toabout 49 percent by weight based on the weight of the copolymer and ispreferably in the range of about 15 to about 40 percent by weight. Athird comonomer can be included, e.g., another alpha-olefin or a dienesuch as ethylidene norbornene, butadiene, 1,4-hexadiene, or adicyclopentadiene. Ethylene/propylene copolymers andethylene/propylene/diene terpolymers are generally referred to as EPRand EPDM, respectively. The third comonomer can be present in an amountof about 1 to 15 percent by weight based on the weight of the copolymerand is preferably present in an amount of about 1 to about 10 percent byweight. Preferably the copolymer contains two or three comonomersinclusive of ethylene.

The LLDPE can include the VLDPE and MDPE, which are also linear, but,generally, has a density in the range of 0.916 to 0.925 g/cc. It can bea copolymer of ethylene and one or more alpha-olefins having 3 to 12carbon atoms, and preferably 3 to 8 carbon atoms. The melt index can bein the range of about 1 to about 20 g/10 min, and is preferably in therange of about 3 to about 8 g/10 min. The alpha-olefins can be the sameas those mentioned above, and the catalysts and processes are also thesame subject to variations necessary to obtain the desired densities andmelt indices.

As noted, included in the definition of polyethylene are homopolymers ofethylene made by a conventional high pressure process. The homopolymerpreferably has a density in the range of 0.910 to 0.930 g/cc. Thehomopolymer can also have a melt index in the range of about 1 to about5 g/10 min, and preferably has a melt index in the range of about 0.75to about 3 g/10 min. Melt index is determined under ASTM D-1238,Condition E. It is measured at 190° C. and 2160 grams.

Peroxide

Although inorganic peroxides can be used in the peroxide used in thepractice of this invention, typically and preferably the peroxide is anorganic peroxide with a decomposition temperature of 100 to 220° C. fora half-life of 10 minutes. Exemplary organic peroxides (with theirdecomposition temperatures in ° C. following in parenthesis) include,but are not limited to, succinic acid peroxide (110), benzoyl peroxide(110), t-butyl peroxy-2-ethyl hexanoate (113), p-chlorobenzoyl peroxide(115), t-butyl peroxy isobutylate (115), t-butyl peroxy isopropylcarbonate (135), t-butyl peroxy laurate (140),2,5-dimethyl-2,5-di(benzoyl peroxy)hexane (140), t-butyl peroxy acetate(140), di-t-butyl diperoxy phthalate (140), t-butyl peroxy maleic acid(140), cyclohexanone peroxide (145), t-butyl peroxy benzoate (145),dicumyl peroxide (150), 2,5-dimethyl-2,5-di(t-butyl-peroxy)hexane (155),t-butyl cumyl peroxide (155), t-butyl hydroperoxide (158), di-t-butylperoxide (160), 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane-3 (170), andalpha,apha′-bis-t-butylperoxy-1,4-diisopropylbenzene (160).

In the practice of the various embodiments of this invention, theperoxide is typically and preferably added to the peroxide-crosslinkablepolymer compositions as a liquid after the compositions have been meltfiltered and formed into pellets. The peroxide is typically sprayed ontothe pellets although alternative forms of application can be employed,e.g., immersion, splashing, etc. The melt-filtered composition,typically in the form of a pellet, is thus impregnated, e.g., soaked,with the peroxide, optionally in combination with one or more additives,e.g., cure co-agents, antioxidants, scorch inhibitors, nitrogenousbases, etc., typically until the pellet is dry to the touch Once theperoxide and any additives are absorbed into the pellet, the pellet isready for packaging. The amount of peroxide applied to and subsequentlyabsorbed by the pellets is such that the pellet typically has a peroxideconcentration of 0.5-2.5 wt %, more typically of 0.5-2.0 wt % and evenmore typically of 0.85-1.9 wt %. In other embodiments peroxide iscompounded into the polymer prior to melt filtration.

Nitrogenous Base

Under acidic conditions peroxide efficiency can decrease with storagetime. Furthermore, water can be generated during the peroxide-initiatedcrosslinking process. The presence of water in the insulation is notdesirable because it can form voids in the material and induce otherconcerns with respect to the material's electrical performance underhigh electrical stress conditions. The decrease of peroxide efficiencywith storage time is not desirable because this imposes a limitation onthe compound's shelf life. WO 99/21194 discloses that the use ofspecific N-substituted hindered amine stabilizers comprised of2,2,6,6-tetramethylpiperidine at concentrations of 0.1 to 0.5 weightpercent can be used to minimize the formation of water withsulfur-containing antioxidants at levels of below 0.15 weight percentwhile maintaining acceptable heat aging performance. Acid(induced/catalyzed) decomposition of the cumyl alcohol generated in theperoxide-initiated crosslinking process can be effectively inhibited byadding a small amount of a material that acts like a base. Whencrosslinking, using the combination of[1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione]and distearylthiodipropionate (DSTDP) with an organic peroxide, the aciddecomposition of the cumyl alcohol can be effectively minimized withvery low levels of a HAS (hindered amine stabilizer), and these levelsare much lower than those discussed in WO 99/21194. The HASconcentration can be effective from levels of 0.002 to 0.1 percent byweight of the polymer. When using a HAS with the combination of[1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione]and DSTDP, the composition is not limited to N-substituted hinderedamine stabilizers comprised of 2,2,6,6-tetramethylpiperidines to haveacceptable heat aging stability. Additionally, this combination of[1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione],DSTDP and very low HAS concentration provides the peroxide containingmaterial with a long shelf life and low moisture generation during cure.

Examples of HAS compounds include, but are not limited to, (i)1,6-hexanediamine, N,N′-bis(2,2,6,6,-tetramethyl-4-piperidinyl)-polymerwith 2,4,6 trichloro-1,3,5triazine, reaction products withN-butyl-1-butanamine and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine;(ii)poly[[6-[(1,1,3,3-tetramethylbutyl)amino]-1,3,5-triazine-2,4-diyl][2,2,6,6-tetramethyl-4-piperidinyl)imino]-1,6-hexanediyl[(2,2,6,6-tetramethyl-4-piperidinyl)imino]]);and (iii) 1,6-hexaneidamine,N,N′-Bis(2,2,6,6-tetramethyl)-4-piperidinyl)-,polymers with2,4-dichloro-6-(4-morpholinyl)-1,3,5-triazine. An alternativedescription of HAS (iii) ispoly[(6-morpholino-s-triazine-2,4-diyl)[2,2,6,6-tetramethyl-4-piperidyl)imino]-hexamethylene[2,2,6,6-tetramethyl-4-piperidyl)imino]]. Other examples of HAScompounds can be found on pages 2 to 8 in Oxidation Inhibition inOrganic Materials by J. Pospisil and P. P. Klemchuk, Volume II. Thenitrogenous bases used in the practice of this invention can be usedalone or in combinations of two or more.

In those embodiments in which the nitrogenous base is added after themelt is filtered, the molecular weight and physical state, e.g., solid,liquid, etc. can vary widely although preferably the base is of lowmolecular weight and/or of a low melting point, or it is liquid. Inthose embodiments in which the nitrogenous base is added prior tofiltering the melt through a micro-screen (less than 100 μm mesh size),the base is of a low molecular weight or of a low melting point, or itis liquid under ambient conditions (i.e., 23° C. and atmosphericpressure). As used in the context of the nitrogenous bases that can beused in the practice of this invention, “low molecular weight” meansnon-polymeric, non-oligomeric and/or of a molecular weight not in excessof 1400 grams per mole (g/mol), preferably not in excess of 1000 g/moland more preferably not in excess of 750 g/mol. As used in the contextof the nitrogenous bases that can be used in the practice of thisinvention, “low melting” means a melting temperature not in excess of95° C., preferably not in excess of 90° C. and more preferably not inexcess of 85° C., as measured by peak melting point using DSC. Examplesof low molecular weight, low melting point, and/or liquid nitrogenousbases that can be used in the practice of this invention includetriallyl cyanurate (TAC), and TINUVIN™ 765 from BASF (mixture ofbis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate and methyl1,2,2,6,6-pentamethyl-4-piperidyl sebacate), TINUVIN™ 770(Bis(2,2,6,6,-tetramethyl-4-piperidyl)sebaceate) from BASF, UVINUL™ 4050(N,N′-bisformyl-N,N′-bis-(2,2,6,6-tetramethyl-4-piperidinyl)-hexamethylendiamine)from BASF, TINUVIN™ 622 (Butanedioic acid, dimethylester, polymer with4-hydroxy-2,2,6,6-tetramethyl-1-piperidine ethanol) from BASF, TINUVIN™123 (Bis-(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl) sebacate) fromBASF, CYASORB™ 3853 ((2,2,6,6-Tetramethyl-4-piperidine) stearate) fromCytec industries.

Although TAC is typically considered as cure booster, its use in thepractice of the present invention is as a nitrogenous base, not as acure booster. As such, the amount of TAC used in the practice of thisinvention is typically well below the amount when used as a curebooster. Typical amounts used in the practice of this invention rangefrom 0.0005 to 0.03 wt %, preferably from 0.002 to 0.01 wt %, based onthe weight of the composition.

Antioxidants

The antioxidants that can be used in the practice of this inventioninclude, but are not limited to, hindered phenols such astetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydro-cinnamate)]methane,bis[(beta-(3,5-ditert-butyl-4-hydroxybenzyl)-methylcarboxyethyl)]-sulphide,and thiodiethylene bis(3,5-di-tert-butyl-4-hydroxy hydrocinnamate);phosphites and phosphonites such astris(2,4-di-tert-butylphenyl)phosphite anddi-tert-butylphenyl-phosphonite; thioesters such asdilaurylthiodipropionate, dimyristylthiodipropionate,distearylthiodipropionate (DSTDP), and pentaerythritol tetrakis(B-laurylthiopropionate); various siloxanes. Additional examples can befound in Plastic Additives Handbook, Gachter et al, 1985. Antioxidantscan be used in amounts of about 0.05 to about 5 percent by weight basedon the weight of the composition. Preferably the composition comprises asulfur-containing antioxidant, e.g. a thioester, especially DSTDP. Theantioxidants can be used alone or in combinations of two or more andwill be referred to as AO.

Additives

Additional additives can be added to the polymer melt before, duringand/or after processing. The amount of additive is usually in the rangeof about 0.01 to about 3 percent by weight based on the weight of thepolymer. Useful additives include additional antioxidants, ultravioletabsorbers, antistatic agents, slip agents, plasticizers, processingaids, lubricants, stabilizers, flow aids, lubricants, water treeinhibitors such as polyethylene glycol, cure boosters, scorchinhibitors, and viscosity control agents.

Pre-Blend Process

In one embodiment the invention is a process for making aperoxide-crosslinkable pellet, the process comprising the steps of firstforming a homogeneous melt of a peroxide-crosslinkable polymer andpre-blend of triallyl cyanurate (TAC) and one or more antioxidants (AO),and then passing the homogeneous melt through a filter with a mesh sizeof less than 100 μm, and then forming pellets from the filteredhomogeneous melt. The pre-blend typically comprises or consistsessentially of 0.6-66 wt %, more typically 0.6-3 wt % and even moretypically 0.6-1.5 wt %, TAC and 34-99.4 wt %, more typically 97-99.4 wt% and even more typically 98.5-99.4 wt %, AO.

Preferred antioxidants are phenolic-based AO, sulfur-based AO, andcombinations of phenolic- and sulfur-based AO. Illustrative examples ofAO useful in this embodiment of the invention include, but are notlimited to, CYANOX™ 1790, TBM6 (4,4′-thio-bis (3-methyl-6tert-butylphenol), TBP6 (2,2′-Thiobis(6-tert-butyl-p-cresol)), IRGANOX1010 (pentaerythritoltetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), IRGANOX 1035(thiodiethylene bis[3-[3,5-di-tert-butyl-4-hydroxyphenyl]propionate]),IRGASTAB KV10 (4,6-bis (octylthiomethyl)-o-cresol), DSTDP(distearylthiodipropionate), DLTDP (dilaurylthiodipropionate) NAUGARD412S (pentaerythritol tetrakis(β-laurylthiopropionate), and combinationsof two or more of these AO. CYANOX products are available from Cytec,IRGANOX and IRGASTAB products are available from BASF, and NAUGARDproducts are available from Addivant. Most preferred AOs are TBM-6 andantioxidant blends described in U.S. Pat. Nos. 6,187,858 and 6,187,847.

Pre-blend can be prepared by methods well known in the art to produce ahomogeneous mixture of chemicals and may include melt blending andsolvent blending.

The peroxide-crosslinkable polymer is typically a polyolefin, and moretypically a polyethylene. The homogenous blend of peroxide-crosslinkablepolymer and pre-blend typically comprises or consists essentially of95-99.9 wt %, more typically 96-98 wt % and even more typically 97-98 wt%, crosslinkable polymer and 0.1-0.6 wt %, more typically 0.2-0.5 wt %and even more typically 0.2-0.4 wt %, pre-blend.

The peroxide-crosslinkable polymer and pre-blend are typically andpreferably mixed in a single or twin screw extruder and then passedthrough a fine mesh screen or filter. The conditions of the compoundingare such to produce a homogenous melt of the polymer and pre-blend,e.g., at a temperature of 140-250° C., or 160-235° C. or 170-225° C. Themesh size of the screen is typically less than 100 microns (μm), moretypically less than 75 μm and even more typically less than 50 μm. Inthe most preferred embodiment it is <25 μm. The composition of thescreen can vary to convenience but is typically metallic, e.g.,stainless steel.

Once the homogenous melt of polymer and pre-blend has passed through thefilter or screen, the melt is removed from the mixer, e.g., extrudedfrom the extruder, and formed into pellets of the desired size andshape. In a further, optional step, the pellets are impregnated with aperoxide, typically by spraying or otherwise applying a liquid peroxideto the pellets and allowing the pellets to absorb the peroxide. In oneembodiment the peroxide is mixed with one or more additives before theperoxide is applied to the pellets. In one embodiment, the additivesinclude additional TAC and/or other low molecular weight or low meltingor liquid nitrogenous bases. Once cooled the pellets, with or withoutabsorbed peroxide, are ready for use, shipping and/or storage.

Low Molecular Weight or Low Melting or Liquid Nitrogenous Base Process

In one embodiment the invention is a process for making aperoxide-crosslinkable pellet, the process comprising first forming ahomogeneous melt of (i) a peroxide-crosslinkable polymer, (ii) a lowmolecular weight, or liquid, or low melting nitrogenous base, and, (iii)one or more antioxidants (AO), and then passing the homogeneous meltthrough a filter with a mesh size of less than 100 μm, and then formingpellets from the filtered homogeneous melt. In one embodiment thehomogenous melt typically comprises or consists essentially of 95-99.9wt %, more typically 96-98 wt % and even more typically 97-98 wt %,peroxide-crosslinkable polymer, and 0.0005-0.09 wt %, more typically0.001-0.03 wt % and even more typically 0.002-0.01 wt %, nitrogenousbase. In one embodiment the homogenous melt typically comprises orconsists essentially of 95-99.9 wt %, more typically 96-98 wt % and evenmore typically 97-98 wt %, peroxide-crosslinkable polymer; 0.0005-0.09wt %, more typically 0.001-0.03 wt % and even more typically 0.002-0.01wt %, nitrogenous base; and 0.01-0.6 wt %, more typically 0.1-0.5 wt %and even more typically 0.1-0.4 wt %, AO.

The peroxide-crosslinkable polymer is typically a polyolefin, and moretypically a polyethylene, and the low molecular weight, or low melting,or liquid nitrogenous base is as described above. In one embodiment thebase is TAC and/or UVINUL™ 4050 from BASF(N,N′-1,6-hexanediylbis(N-(2,2,6,6-tetramethyl-4-piperidinyl)-formamide).The optional antioxidant is as described above. Preferred AO includeCYANOX™ 1790, TBM6, TBP6 (2,2′-Thiobis(6-tert-butyl-p-cresol)), IRGANOX1010, IRGANOX 1035, DSTDP, DLTDP, NAUGARD™ 412S, and combinations of twoor more of these AO. In the preferred embodiment the compositioncontains at least one sulfur-containing AO, e.g., DSTDP, DLTDP andNAUGARD™ 412S.

The operational steps of and equipment used in (1) forming thehomogeneous melt of (A) peroxide-crosslinkable polymer, (B) lowmolecular weight, or liquid, or low melting nitrogenous base, and (C)AO, (2) filtering the homogeneous melt, and (3) forming pellets from thefiltered melt are the same as described above for the pre-blend process.

In a further, optional step, the pellets are impregnated with aperoxide, typically by spraying or otherwise applying a liquid peroxideto the pellets and allowing the pellets to absorb the peroxide. In oneembodiment the peroxide is mixed with one or more additives before theperoxide is applied to the pellets. In one embodiment, the additivesinclude additional TAC and/or TINUVIN™ 765 and/or other low molecularweight, or liquid, or low melting nitrogenous bases at 0.0005% to 0.01%.Once cooled the pellets, with or without absorbed peroxide, are readyfor use, shipping and/or storage.

Soaking Process

In one embodiment the invention is a process for making aperoxide-crosslinkable pellet, the process comprising first forming amelt of a peroxide-crosslinkable polymer and then passing the meltthrough a filter with a mesh size of less than 100 μm, and then formingpellets from the filtered homogeneous melt, and then impregnating thepellets with a low molecular weight, or a low melting, or liquidnitrogenous base. The peroxide-crosslinkable polymer is typically apolyolefin, and more typically a polyethylene, and it may be combinedwith one or more additives, e.g., antioxidants such as those describedabove. The operational steps of and equipment used in (1) forming themelt of the peroxide-crosslinkable polymer with or without additives,(2) filtering the melt, and (3) forming pellets from the filtered meltare the same as described above for the pre-blend and low molecularweight or low melting or liquid nitrogenous base processes.

The low molecular weight, or low melting, or liquid nitrogenous base isas described above. In one embodiment the base is TAC and/or TINUVIN™765 and/or other low molecular weight, or liquid, or low meltingnitrogenous bases at 0.0005% to 0.01%. The optional antioxidant is alsoas described above. Preferred AO include CYANOX™ 1790, TBM6, TBP6(2,2′-Thiobis(6-tert-butyl-p-cresol)), IRGANOX 1010, IRGANOX 1035,DSTDP, DLTDP, NAUGARD™ 412S, and combinations of two or more of theseAO. In the preferred embodiment the composition contains at least onesulfur-containing AO, e.g., DSTDP, DLTDP and NAUGARD™ 412S. Mostpreferred AOs are TBM-6 and antioxidant blends described in U.S. Pat.Nos. 6,187,858 and 6,187,847.

The pellets are then impregnated with a low molecular weight, or a lowmelting, or liquid nitrogenous base, typically and preferably incombination with a peroxide, by spraying or otherwise applying the baseand optional peroxide to the pellets and allowing the pellets to absorbthe base and optional peroxide. The base and peroxide are liquid at thetime and temperature of their application to the pellets. In oneembodiment the base is mixed with the peroxide and/or one or moreadditives before it is applied to the pellets. In one embodiment thepellet, after application of the low molecular weight, or low melting,or liquid base, typically comprises or consists essentially of 95-99.9wt %, more typically 96-98 wt % and even more typically 97-98 wt %,peroxide-crosslinkable polymer, and 0.001-0.09 wt %, more typically0.001-0.03 wt % and even more typically 0.002-0.01 wt %, nitrogenousbase. In one embodiment the pellet, after application of the lowmolecular weight, or low melting, or liquid base, typically comprises orconsists essentially of 95-99.9 wt %, more typically 96-98 wt % and evenmore typically 97-98 wt %, peroxide-crosslinkable polymer; 0.0005-0.09wt %, more typically 0.001-0.03 wt % and even more typically 0.002-0.01wt %, nitrogenous base; and 0.01-0.6 wt %, more typically 0.1-0.5 wt %and even more typically 0.1-0.4 wt %, AO. In one embodiment thecomposition comprises a sulfur-containing antioxidant, e.g., DSTDP.

Once cooled the pellets, with or without absorbed peroxide, are readyfor use, shipping and/or storage.

Wire and Cable

The peroxide-crosslinkable polymer compositions of this invention can beapplied to a cable as an insulation known amounts and by known methods(for example, with the equipment and methods described in U.S. Pat. Nos.5,246,783 and 4,144,202). Typically, the sheath composition is preparedin a reactor-extruder equipped with a cable-coating die and after thecomponents of the composition are formulated, the composition isextruded over one or more conductors as the cable is drawn through thedie.

EXAMPLES

Stabilized Pre-Blend

The inventive examples use a mixture of a primary antioxidant, CYANOX™1790 from Cytec (1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione), and asynergistdistearylthiodipropionate (DSTDP) blended with triallylcyanurate (TAC). The ratio of the primary AO and synergist in allexamples is about 0.63:1.

Example 1

Example 1 is a 1:1 pre-blend of TAC and AO mixture (50 wt % TAC+31 wt %DSTDP+19 wt % CYANOX™ 1790). Differential scanning calorimetry (DSC)thermograms of all samples are shown in the figures. DSC experiments arerun in air with 10° C./min ramp rate. Peak onset, peak maximum andenthalpy values for these thermograms are collated in Table 1. For theAO mixture without TAC (61 wt % DSTDP+39 wt % CYANOX™ 1790), there aretwo main peaks: a sharp endotherm at about 60° C. (peak 1) and a broadexotherm between 245 and 285° C. (peak 2). Thermogram for TAC (no AO)also shows two peaks, a sharp exotherm at ˜200° C. (peak 3) and a broadexotherm between 255° C. and 295° C. (peak 4).

When the AO mixture and TAC are pre-blended in a 1:1 ratio in Example 1,surprisingly, no trace of peak 3 is observed, while there is nosignificant effect (apart from dilution) on the other peaks (1, 2, and4). Example 1 therefore only has two peaks: peak 5 which comes from theAO mixture (similar to peak 1) and peak 6 which is a combination ofpeaks 2 and 4. This result is clearly shown in FIG. 1 where all threethermograms are collated. Peak 3 in the TAC thermogram is a highlyexothermic sharp peak indicating a safety hazard and a potential forexplosion. The inventive mixture of Example 1 completely annihilatesthis peak (reaction), significantly improving the safety. Hence, thehandling and introduction of the AO and TAC as a pre-blend is safer thanhandling and introducing the components individually.

TABLE 1 Peak Descriptions, Onsets, Maximums, and Enthalpies of AOs, TAC,Tetradecane and Their Mixtures Peak onset Peak max Enthalpy CompositionPeak # Peak Energy Peak Type (C.) (C.) (J/g) Control Sample 61% DSTDP +39% Cyanox 1790 mixture 1 endotherm Sharp 59.2 61.7 137.1 2 exothermbroad 244.2 284.6 202.4 Control sample TAC 3 exotherm Sharp 201.8 205.6274.2 4 exotherm broad 258.9 293.5 237.0 Inventive 50% TAC + 31% DSTDP +19% 5 endotherm Sharp 57.1 65.3 87.8 example 1 Cyanox 1790 mixture 6exotherm broad 254.7 281.2 178.1 Inventive 2% TAC + 60% DSTDP + 38% 7endotherm Sharp 60.3 62.5 107.9 example 2 Cyanox 1790 mixture 8 exothermbroad 249.8 286.0 87.2 Comparative 50% TAC + 50% Tetradecane 9 exothermSharp 204.7 208.7 178.9 Sample 10 exotherm broad 272.3 300.0 68.4Control Sample Tetradecane 11 exotherm broad 191.3 205.9 75.1 12endotherm broad 221.0 266.1 180.7

Example 2

The composition of Example 2 is 2 wt % TAC+60 wt % DSTDP+38 wt % CYANOX™1790. This blend is at TAC levels that will not significantly alter thecure characteristics of the formulation in which the AO+TAC pre-blend isadded. This is within the preferred compositional range. Example 1clearly demonstrates the effect of AOs in eliminating one of theexothermic reactions in TAC. FIG. 2 shows the elimination of peak 3 inthe second composition as well. Peak onset, peak maximum and enthalpyvalues for these thermograms are collated in Table 1.

COMPARATIVE AND CONTROL EXAMPLES

A unique feature of this invention is that elimination of an exothermicpeak is not a universal effect in all TAC+solvent blends. To prove this,DSC experiments are conducted on a comparative formulation similar tothat used in Example 1 except tetradecane is substituted for the AOmixture: 50 wt % tetradecane+50 wt % TAC. Control experiments are alsorun on a neat tetradecane sample. FIG. 3 compares the thermograms of theComparative Example to that of TAC and tetradecane, and the exothermicTAC peak (peak 3) is clearly observed in the mixture as peak 9. Peakonset, peak maximum and enthalpy values for these thermograms arecollated in Table 1.

Compositions Comprising Low Molecular Weight Nitrogenous Bases

In this embodiment the invention is a composition comprising lowmolecular weight, liquid or low melting organic nitrogenous bases(subsequently referred to as low molecular weight or low melting bases)along with sulfur-based antioxidants. Examples of such bases are givenin Table 2. Preferred examples of such bases include UVINUL™ 4050 andtriallyl cyanurate (TAC). CYASORB™ 3346 and CHIMASSORB™ 2020 arecomparative examples, with both high molecular weights and high meltingpoints.

TABLE 2 Examples of Low and High Molecular Weight Nitrogenous BasesMolecular Weight Melting Point (° C.) TAC 249 28 Uvinul 4050 450 158Tinuvin 622 3100-5000 60 Tinuvin 765 509 20 Tinuvin 123 737 <20 Cyasorb3853 438 30 Tinuvin 770 481 85 Cyasorb 3346 1600-1700 100 Chimassorb2020 2600-3400 120-150

UVINUL™ 4050 from BASF(N,N′-1,6-hexanediylbis(N-(2,2,6,6-tetramethyl-4-piperidinyl)-formamide)is a low molecular weight base, and TAC has both a low molecular weightand a low melting point. On the other hand CYASORB™ 3346 from Cytec(Poly[[6-(4-morpholinyl)-1,3,5-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidinyl)imino]-1,6-hexanediyl-[(2,2,6,6-tetramethyl-4-piperidinyl)imino]])and CHIMASSORB™ 2020 from BASF (1,6-Hexanediamine,N,N′-bis(2,2,6,6-tetramethyl-4-piperidinyl)-polymer with2,4,6-trichloro-1,3,5-triazine, reaction products withN-butyl-1-butanamine and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine)have both high molecular weight and high melting point. TINUVIN™ 765from BASF (mixture of bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate andmethyl 1,2,2,6,6-pentamethyl-4-piperidyl sebacate) is a liquid base.

When low molecular weight or low melting bases are used in formulationscontaining sulfur based antioxidants, surprisingly, there is asignificant reduction in screen buildup during the production of theseformulations in an extruder. Experiments are run on a ZSK-30 twin screwextruder to simulate the screen build up. Additive loadings used inthese experiments are: 1.37 wt % DSTDP+0.83 wt % CYANOX™ 1790+344 partsper million (ppm) of base. These loadings are nearly six times of whatis used in the preferred embodiment, and are used to accelerate thescreen build up. In other words, these tests with 344 ppm of basesimulate the performance expected when using approximately 50 ppm of thesame bases in an actual application. Experiments are run at 245° C.Stacks of screens ranging from 23 μm openings to 200 μm openings areused for each case, i.e. in each case, the finest screen present had 23μm openings. Post extrusion, the screen packs are delaminated, polymeris removed by toluene wash, and plate out is measured by immersing the23 μm opening screen in water and weighing the water soluble extract.Table 3(a) shows the build up on the screens in terms of milligrams (mg)of water soluble extract for various formulations. Build up of all thelow molecular weight or low melting buffers is lower than that ofCYASORB™ 3346.

TABLE 3(a) Inventive, Preferred, and Comparative Examples, Screen Buildup Measured as Water-Soluble Extract in mg Build up, mg of water solubleextract on the Formulation in LDPE screens Inventive Preferred 1.37%DSTDP + 0.83% C1790 + 0.0344% TAC 0.6 examples examples 1.37% DSTDP +0.83% C1790 + 0.0344% Uvinul 4050 0.4 1.37% DSTDP + 0.83% C1790 +0.0344% Tinuvin 622 0.6 1.37% DSTDP + 0.83% C1790 + 0.0344% Tinuvin 7650.5 1.37% DSTDP + 0.83% C1790 + 0.0344% Tinuvin 123 0.4 1.37% DSTDP +0.83% C1790 + 0.0344% Cyasorb 3853 0.6 1.37% DSTDP + 0.83% C1790 +0.0344% Tinuvin 770 0.3 Comparative example 1.37% DSTDP + 0.83% C1790 +0.0344% Cyasorb 3346 1.1

Less screen build up correlates to longer run times in the productioncycles and hence the higher yields of extra clean product for extrahigh-voltage applications. As the screen starts to build up, thepressure builds up at the breaker plate until it reaches a level whereit becomes ineffective to operate. As shown in Table 3(b) where pressurerise data from actual plant runs are presented, when the formulations ofthe Comparative Examples (containing 0.225% DSTDP+0.137% CYANOX™1790+0.0075% CYASORB™ 3346) and (containing 0.225% DSTDP+0.137% CYANOX™1790+0.0075% CHIMASSORB™ 2020) are run, the average rate of pressurerise is 0.7 bar/hour. This would lead to run lengths of less than 3 daysbefore reaching a point where it becomes ineffective to operate. Smallimprovements can be made to the comparative examples by reducing theloadings of the additives. A formulation containing 0.200% DSTDP+0.120%CYANOX™ 1790+0.0022% CYASORB™ 3346 gave a pressure rise of 0.5 bar/hourwhich would make the run length just over four days. Even at this runlength, however, very small yields of extra clean product can beobtained. On the other hand, our inventive (preferred) examplescontaining (0.200% DSTDP+0.120% CYANOX™ 1790+0.0030% UVINUL™ 4050) and(0.200% DSTDP+0.120% CYANOX™ 1790+0.0050% TAC) give no pressure rise inthe plant, thus giving very long run lengths and very good yields of theextra clean product.

TABLE 3(b) Rate of Pressure Rise from Commercial Plant Runs forCompounded High Molecular Weight Bases (Comparative) and Low MolecularWeight and/or Low Melting Nitrogenous Bases (Preferred Inventive)Present during Extrusion Pressure rise during Additive composition inperoxide crosslinkable extrusion Example polymer (bar/hr) Inventive,0.200% DSTDP + 0.120% Cyanox 1790 + 0.0 Preferred 0.0030% Uvinul 4050Inventive, 0.200% DSTDP + 0.120% Cyanox 1790 + 0.0 Preferred 0.0050% TACComparative 0.200% DSTDP + 0.120% Cyanox 1790 + 0.5 0.0022% highmolecular weight base Cyasorb 3346 Comparative 0.225% DSTDP + 0.137%Cyanox 1790 + 0.7 0.0075% high molecular weight base Cyasorb 3346Comparative 0.225% DSTDP + 0.137% Cyanox 1790 + 0.7 0.0075% highmolecular weight base Chimassorb 2020

There are two main advantages that a base imparts to the sulfur-based AOcontaining formulations: (1) it provides storage stability to theperoxide, and (2) it mitigates the undesirable generation of waterduring cure. To measure the peroxide stability, samples are placed in anoven at 70° C. and the cure potential (MDR-Mh) of these is analyzedperiodically. A base is considered effective in maintaining the storagestability of peroxide if there is more than 90% retention in cure(retention of initial Mh) after two weeks of aging at 70° C. This isused to provide prediction of shelf life at room temperature. Table 4shows the MDR-Mh values of the inventive, preferred, comparative, andcontrol samples over two weeks of aging. Based on the percentage changein Mh of aged sample from the initial, samples are assigned a pass orfail rating. Samples in the table are compounded in a pilot plant on aZSK-30 twin screw extruder and subsequently impregnated (soaked) withperoxide. All samples contained 0.20 wt % DSTDP+0.12 wt % CYANOX™1790+1.8 wt % dicumyl peroxide (Dicup) and 0.0050 wt % of the respectivebuffer.

TABLE 4 Inventive, Preferred, Comparative, and Control Samples: PeroxideStability Measured by Retention of Cure MDR-Mh @ 182 C. after t days %Mh of aging at 70 C. retention at Peroxide Formulation in LDPE t = 0 t =2 t = 7 t = 14 14 days stability Preferred example 1 0.20% DSTDP + 0.12%C1790 + 0.005% TAC 2.60 2.60 2.56 2.50 96 Pass Preferred example 2 0.20%DSTDP + 0.12% C1790 + 0.005% Uvinul 4050 2.66 2.71 2.71 2.72 102 PassInventive examples 0.20% DSTDP + 0.12% C1790 + 0.005% Tinuvin 622 2.532.66 2.65 2.61 103 Pass 0.20% DSTDP + 0.12% C1790 + 0.005% Tinuvin 7652.65 2.66 2.72 2.66 100 Pass 0.20% DSTDP + 0.12% C1790 + 0.005% Tinuvin123 2.66 2.61 2.66 2.61 98 Pass 0.20% DSTDP + 0.12% C1790 + 0.005%Cyasorb 3853 2.77 2.66 2.69 2.75 99 Pass 0.20% DSTDP + 0.12% C1790 +0.005% Tinuvin 770 2.57 2.68 2.74 2.67 104 Pass Comparative example0.20% DSTDP + 0.12% C1790 + 0.005% Cyasorb 3346 2.51 2.72 2.67 2.25 90Pass Control 0.20% DSTDP + 0.12% C1790 2.97 2.97 2.46 0.62 21 Fail

Among the inventive formulations with bases that minimize screen buildup and provide good peroxide stability, the ones that give the optimumcombination of other properties are TAC and UVINUL™ 4050, and hence arethe preferred bases in this invention. Table 5 compares the electricalproperties and the effectiveness of the base in mitigating watergeneration post curing for our preferred, inventive, and comparativeexamples. Comparison of electrical and buffer properties is made forvarious buffers in formulations containing 0.2 wt % DSTDP, 0.12 wt %CYANOX™ 1790, 1.8 wt % Dicup and 0.005 wt % buffer. TINUVIN™ 765,TINUVIN™ 123, CYASORB™ 3853, and TINUVIN™ 770 do not adequately mitigateundesirable water generation. The samples with DSTDP, Cyanox 1790 and nobase are expected to give moisture values higher than all the entries inTable 5. Hence df measurements were not performed on TINUVIN™ 123,CYASORB™ 3853, and TINUVIN™ 770 containing formulations. TINUVIN™ 622and TINUVIN™ 765 give high df values at high temperature (130° C.) andhigh stresses (20 kV/mm), which is undesirable for high voltageinsulation applications. The df measurements are made on model cables.Preferred examples, TAC and UVINUL™ 4050 provide significanteffectiveness both in terms of peroxide stability and mitigation ofwater generation, and have low df value at high temperature and highstress, while giving low level of screen build up.

TABLE 5 Preferred, Inventive, and Comparative Examples Water generationin df @ 98 C., df @ 130 C., Karlfischer at 20 Kv/mm 20 KV/mmFormulations In LDPE 240 C., (ppm) (%) (%) Preferred example 1 0.20%DSTDP + 0.12% C1790 + 0.005% TAC 49 0.027 0.111 Preferred example 20.20% DSTDP + 0.12% C1790 + 0.005% Uvinul 4050 33 0.01 0.091 Inventiveexamples 0.20% DSTDP + 0.12% C1790 + 0.005% Tinuvin 622 76 0.028 0.2110.20% DSTDP + 0.12% C1790 + 0.005% Tinuvin 765 219 0.017 0.222 0.20%DSTDP + 0.12% C1790 + 0.005% Tinuvin 123 470 0.20% DSTDP + 0.12% C1790 +0.005% Cyasorb 3853 463 0.20% DSTDP + 0.12% C1790 + 0.005% Tinuvin 770219 Comparative example 0.20% DSTDP + 0.12% C1790 + 0.005% Cyasorb 334630 0.023 0.094

Use of TAC as a coagent for cure boosting is known, but the loadingsused in this invention are well below the cure boosting levels. Table 6shows that there is no significant difference between the MDR-Mh values(cure levels) of Example 1 (with 0.005 wt % TAC) and the control(without any TAC). Both formulations had 1.8 wt % Dicumyl peroxide.Thus, the use of TAC at these levels is not obvious in view of the artthat teaches its use as a coagent (i.e., cure booster).

TABLE 6 Cure Levels (MDR-Mh at 182° C.) of Example 1 with 0.005 wt % TACCompared to Same Formulation without TAC Formulation MDR-Mh (lb.-in.)0.20% DSTDP + 0.12% C1790 + 0.0050% TAC 2.60 0.20% DSTDP + 0.12% C17902.66Soaking Process

The baseline compositions in the inventive and comparative examples ofthis embodiment of the invention contain two main antioxidants: CYANOX™1790 and DSTDP compounded in LDPE. One version of this formulation ismade in a production plant and contains 0.137 wt % CYANOX™ 1790 and0.225 wt % DSTDP (i.e., baseline formulation 1). Another version of thisformulation is made in a production plant and contains 0.12 wt % CYANOX™1790 and 0.20 wt % DSTDP (i.e., baseline formulation 2). Other versionsof the baseline formulation are made at pilot scale on a ZSK-30 twinscrew extruder with different AO loadings.

Experiments are conducted in which various nitrogenous bases aredissolved in dicumyl peroxide (Dicup) at 60° C., and pellets of baselineformulation 1 are soaked with the resulting liquid mixture. Aftersoaking the mixture of Dicup (1.8 wt %) and base (0.005 wt %) intobaseline formulation 1 (98.195 wt %), the ability of the base to provideperoxide stability during storage and mitigate water formation duringcure (the two properties referred to hereinafter as buffering) isassessed. Table 7 summarizes the bases in peroxide-base mixtures used inthe examples.

TABLE 7 Solubility of Bases in Dicup at 60° C. Solubility in Dicup at 60C., Molecular Melting with Dicup:base Weight Point (° C.) ratio 360:1Tinuvin 765 509 20 soluble Tinuvin 770 481 85 soluble TAC 249 28 solubleTinuvin 622 3100-5000 60 soluble Tinuvin 123 737 <20 soluble Cyasorb3853 438 30 soluble Cyasorb 3346 1600-1700 100 not soluble Chimassorb2020 2600-3400 120-150 not soluble

Bases CYASORB™ 3346 from Cytec(poly[[6-(4-morpholinyl)-1,3,5-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidinyl)imino]-1,6-hexanediyl[(2,2,6,6-tetramethyl-4-piper-idinyl)imino]])and CHIMASSORB™ 2020 from BASF(N,N′-bis(2,2,6,6-tetramethyl-4-piperidinyl)-1,6-hexanediamine polymerwith 2,4,6-trichloro-1,3,5-triazine reaction products withN-butyl-1-butanamine and N-butyl-2,2,6,6-tetramethyl-4-piperidinamine)are high molecular weight solid hindered amine light stabilizers (HALS).Since these do not dissolve in dicumyl peroxide (Dicup) at 60° C., theseare not introduced to the pellets by the soaking method.

TINUVIN™ 622 from BASF (an oligomeric hindered amine; butanedioc acid,dimethylester, polymer with 4-hydroxy-2,2,6,6-tetramethyl-1-piperidineethanol), a high molecular weight low melting HALS, though soluble inDicup, is not very effective as a base (for buffering) when introducedvia soaking. Control experiments are run where the same loading ofTINUVIN™ 622 (0.005 wt %) is introduced via compounding in a singlescrew extruder at 200° C., and good buffering properties are observed. Agood base provides stability to the peroxide and maintains the curelevels measured by MDR-Mh. Table 8 compares the MDR-Mh values over fourweeks of aging at 70° C. for TINUVIN™ 622 introduced via compounding andsoaking. FIG. 4 presents this cure retention data by normalizing theMDR-Mh values of an aged sample with its day zero (initial) MDR-Mh.Table 8 and FIG. 4 clearly indicate that TINUVIN™ 622 when introducedvia soaking, leads to loss in cure faster than when introduced viacompounding. Hence, surprisingly, low molecular weight bases arerequired for soaking to be effective and enable one to circumvent thescreen build up encountered with prior art high molecular weight highmelting HALS.

TABLE 8 Comparative Examples MDR-Mh Values of TINUVIN ™ 622 Introducedin Baseline Formulation 1 by Soaking with Dicup and Compounding AllFormulations had 1.8% Dicup. Samples aged at 70° C. MDR-Mh @ 182 C.after t days of aging at 70 C. t = 0 t = 3 t = 7 t = 13 t = 21 t = 28day day day day day day TINUVIN 622 2.61 2.67 2.64 2.61 2.28 0.20compounded in baseline formulation 1 Tinuvin 622 soaked in 2.71 2.612.19 1.74 1.30 0.00 baseline formulation 1 Baseline formulation 1 2.662.47 1.70 0.41 0.00 0.00

A base is considered effective in maintaining the storage stability ofperoxide if there is more than 90% retention in cure (retention ofinitial Mh) after two weeks of aging at 70° C. Based on this criterion,the tabulated performance of TINUVIN™ 622 soaked (fail) and compounded(pass) in Table 9.

TABLE 9 Comparative Examples. Effectiveness of TINUVIN ™ 622 as BaseWhen Introduced by Soaking and Compounding. Day 14 Values are Calculatedby Interpolating between Day 13 and Day 21. MDR-Mh @ % Mh 182 C. after tretention Peroxide days of aging at 70 C. at 14 storage t = 0 days t =13 day days stability Tinuvin 622 2.61 2.61 98% Pass compounded inbaseline formulation 1 Tinuvin 622 soaked in 2.71 1.74 62% Fail baselineformulation 1 Baseline formulation 1 2.66 0.41 13% Fail

The above results establish that high molecular weight bases do notprovide effective peroxide storage stability via soaking. As discussedbelow and as is known in the art, although high molecular weight basesdo provide peroxide storage stability when compounded prior toscreening, this results in unacceptable screen fouling.

As demonstrated below in data from pilot scale experiments, introducinglow molecular weight base via soaking (post extrusion) surprisinglygives significant reduction in screen build up in the extruder (comparedto when high as well as low molecular weight bases are introduced bycompounding), provides effective peroxide storage stability, andmitigates water generation post cure compared to soaked high molecularweight bases. Furthermore, when post-extrusion soaking of low molecularweight bases is implemented in the commercial plants, it surprisinglyreduces the rate of pressure build up, thus allowing long run times forproducing good yields of extra clean product.

Pilot plant experiments are run on a ZSK-30 twin screw extruder tosimulate the screen build up in an actual production plant. Higheradditive loadings (six times the baseline formulation 1 loadings: 1.37wt % DSTDP+0.83 wt % CYANOX™ 1790) are used for these experiments toaccelerate the screen build up. Experiments are run at 245° C. A stackof screens ranging from 23 μm opening to 200 μm opening is used.Post-extrusion, the screen packs are delaminated, polymer is removed bytoluene wash, and build up is measured by immersing the 23 μm openingscreen in water and weighing the water soluble extract. Table 10 showsthe build up on the screens in terms of mg of water soluble extract forinventive (post-extrusion soaked lower molecular weight bases) andcomparative (compounded) examples.

TABLE 10 Build up on the Screens Measured as Water-Soluble Extract in mgfor Formulations Run in Pilot Plant on Twin Screw ZSK-30 Extruder mg ofFormulation extract Post Extrusion 1.37% DSTDP + 0.83% C1790 + 0.0344%0.15 Soaked low TAC (TAC soaked post extrusion) mwt bases 1.37% DSTDP +0.83% C1790 + 0.0344% 0.15 (inventive) T765 (T765 soaked post extrusion)Compounded 1.37% DSTDP + 0.83% C1790 + 0.60 bases 0.0344% TAC compounded(comparative) 1.37% DSTDP + 0.83% C1790 + 0.50 0.0344% T765 compounded1.37% DSTDP + 0.83% C1790 + 0.0344% 1.10 C3346 compounded

Less screen build up correlates to longer run times in the productioncycles and hence the higher yields of extra clean product for extra highvoltage applications. As the screen starts to build up, the pressurebuilds up at the breaker plate until it reaches a level where it becomesinefficient to operate. Table 11 shows that when baseline formulation 1and baseline formulation 2 (containing only DSTDP and CYANOX™ 1790 andno base) are run through the extruder in an actual plant there is nopressure rise. Base is added to these formulations post-extrusion bysoaking. On the other hand, when similar formulations with highmolecular weight bases (0.225 wt % DSTDP+0.137 wt % CYANOX™ 1790+0.0075wt % CYASORB™ 3346), (0.225 wt % DSTDP+0.137 wt % CYANOX™ 1790+0.0075 wt% CHIMASSORB™ 2020) and (0.225 wt % DSTDP+0.137 wt % CYANOX™ 1790+0.0022wt % CYASORB™ 3346) are run, the average rate of pressure rise was 0.7bar/hour, 0.7 bar/hour, and 0.5 bar/hour, respectively. This clearlyshows the advantage of the current inventive process.

TABLE 11 Rate of Pressure Rise for Compounded High Molecular WeightBases (Comparative) and for No Base Present during Extrusion Pressurerise during Additive composition in peroxide crosslinkable extrusionExample polymer (bar/hr) Inventive 0.225% DSTDP + 0.137% C1790 (low 0.0molecular weight base added after extrusion) Inventive 0.200% DSTDP +0.120% C1790 (low 0.0 molecular weight base added after extrusion)Comparative 0.200% DSTDP + 0.120% Cyanox 1790 + 0.5 0.0022% highmolecular weight base Cyasorb 3346 Comparative 0.225% DSTDP + 0.137%Cyanox 1790 + 0.7 0.0075% high molecular weight base Cyasorb 3346Comparative 0.225% DSTDP + 0.137% Cyanox 1790 + 0.7 0.0075% highmolecular weight base Chimassorb 2020

Additional surprising advantage of introducing the bases through soakingis the reduced heat history for the base, since apparently, based on thefollowing data, extra heat history during compounding can negativelyaffect the properties of the formulation. Table 12 shows that electricaland buffering properties of TINUVIN™ 765 (a low molecular weight base)at the same loading are dramatically improved when introduced viasoaking as compared to compounding. High temperature high stressdissipation factor and water generation post curing were significantlylower for the soaked TINUVIN™ 765 based formulation compared to thecompounded version of the same formulation. Along with peroxidestability during storage, the bases also mitigate the undesirable watergeneration during cure. Water generation in the soaked TINUVIN™ 765sample is much lower than that in the compounded version showing betterbuffering through soaking. The samples with DSTDP, Cyanox 1790 and nobase are expected to give moisture values higher than all the entries inTable 12.

TABLE 12 Inventive and Comparative Examples. Comparison of Electrical(Dissipation Factor) and Buffering Properties of Triallyl Cyanurate(TAC) and TINUVIN ™ 765 Introduced in the Formulations by Soaking andCompounding Peroxide stability, Water df @ 98 C. pass if in 14 days atgeneration in 20 kV/mm df @ 130 C. 20 kV/mm 70 C., there is >90%Karlfischer at Formulation (%) (%) retention of cure 240 C. (ppm)Inventive Example 1 0.20% DSTDP + 0.12% C1790 + 0.0050% 0.029 0.128 pass55 TAC soaked Inventive Example 2 0.20% DSTDP + 0.12% C1790 + 0.0050%0.014 0.125 pass 39 T765 soaked Comparatve Example 1 0.20% DSTDP + 0.12%C1790 + 0.0050% 0.027 0.111 pass 49 TAC compounded Comparatve Example 20.20% DSTDP + 0.12% C1790 + 0.0050% 0.017 0.222 pass 219 T765 compoundedComparatve Example 3 0.20% DSTDP + 0.12% C1790 + 0.0050% 0.023 0.094pass 30 C3346 compounded All formulations in Table 6 had 1.8% Dicup.

Use of TAC as a coagent for cure boosting is known in the art, howeverthe loadings claimed in this invention are well below the cure boostinglevels. Table 13 shows that there is no difference between the MDR-Mhvalues (cure levels) of Example 1 with 0.005 wt % soaked TAC and thecontrol without any TAC. Both formulations had 1.8 wt % dicumylperoxide. Thus, the use of TAC at these levels is not suggested by theart.

TABLE 13 Cure Levels (MDR-Mh at 182 C.) of Example 1 with 0.005 wt %Soaked TAC Compared to Same Formulation without TAC MDR-Mh (lb.-Formulation in LDPE in.) 0.20% DSTDP + 0.12% C1790 + 0.0050% 2.65 soakedTAC 0.20% DSTDP + 0.12% C1790 2.66

What is claimed is:
 1. A process for making a peroxide-crosslinkablepellet, the process comprising the steps of: (1) forming a homogeneousmelt of: (A) a peroxide-crosslinkable polymer; (B) a low molecularweight, or low melting, or liquid nitrogenous base, and (C) optionally,an antioxidant (AO); (2) passing the homogeneous melt of (1) through afilter with a mesh size of less than 100 μm; and (3) forming pelletsfrom the filtered homogeneous melt of (2).
 2. The process of claim 1comprising the further step of impregnating the pellets with a peroxide.3. The process of claim 1 in which the antioxidant comprisestetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydro-cinnamate)]methane;4,4′-thiobis-(3-methyl-6-t-butylphenol); and distearylthiodipropionate(DSTDP).
 4. The process of claim 1 in which the antioxidant comprises[1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione];and distearylthiodipropionate (DSTDP).
 5. A wire or cable comprising asheath made by the process of claim 1.